1. Field of the Invention
The present invention relates to a laser apparatus such as a metal-vapor laser apparatus, and particularly to a laser apparatus which can perform stable and high-output laser oscillation.
2. Description of the Prior Art
As a known laser apparatus, for example, there are a metal-vapor laser, a metal-ion laser, an excimer laser and a TEA CO.sub.2 laser. Of these, for example, a conventional metal-vapor laser apparatus of a transverse excitation type is so constructed as shown in FIG. 1. As shown in FIG. 1, in the conventional metal-vapor laser apparatus of a transverse excitation type, a discharge tube 1 being made of ceramics excellent in heat resistance is inserted in a protective tube 3 in which a thermal insulating material 2 is filled. The protective tube 3 is inserted in a vacuum container 4 comprising a conductive body. Moreover, the discharge tube 1 is supported by the vacuum container 4 through supporting material 5a, 5b, and the protective tube 3 is supported by the vacuum tube 4 through O rings 6a, 6b.
In the discharge tube 1, an anode 7 and a cathode 8 respectively consisting of molybdenum or the like are arranged to oppose each other along the axial direction. The anode 7 and the cathode 8 are integrally secured by screws 10 together with the discharge tube 1 to current introducing plates 9a, 9b consisting of niobium, molybdenum or the like and extending along the axial direction in the protective tube 3. Namely, the discharge tube 1 and the anode 7 or the cathode 8 are made with different materials. Moreover, in the discharge tube 1, several lumps of metal 11 are placed. Each one end of the current introducing plates 9a, 9b is connected to the vacuum container 4 through current introducing terminals 12a, 12b respectively so that a high-voltage pulse from a pulse power source 13 is applied to the space between the anode 7 and the cathode 8 through the current introducing terminals 12a, 12b and the current introducing plates 9a, 9b in the vacuum container 4.
Between the protective tube 3 and the vacuum container 4, a vacuum thermal insulating chamber 14 is provided to be exhausted by a rotary pump (not shown) when used. Moreover, an insulating tube 15 is disposed in the vacuum container 4 so as to electrically insulate the anode 7 and cathode 8, and the right side thereof shown in the drawing is for the anode 7 and the left side is for the cathode 8.
At both the end openings of the vacuum container 4, windows 16a, 16b are attached, and at both the outsides thereof are disposed a total reflection mirror 17 and an output mirror 18. The interior of the vacuum container 4 sealed up by the attachment of the windows 16a, 16b at both the ends is exhausted by a rotary pump (not shown) to be kept in a vacuum state, and is fed with a buffer gas for discharge, such as He or Ne, from a gas feeding source (not shown).
To carry out laser oscillation by the conventional metal-vapor laser apparatus of a transverse exciting type having the construction as stated above, the vacuum container 4 is exhausted by a rotary pump (not shown) to be kept in a high-vacuum state, then is fed with a buffer gas for discharge, such as He, Ne or the like, from a gas feeding source (not shown). Thereafter, several lumps of metal 11 are placed in the discharge tube 1, and a high-voltage pulse is applied from the pulse power source 13 to the space between the anode 7 and the cathode 8 through the vacuum container 4, the current introducing terminals 12a, 12b, and the current introducing plates 9a, 9b. Then, discharge is generated between the anode 7 and the cathode 8 to elevate the temperature in the discharge tube 1 by Joule heating, then the lumps of metal 11 are vaporized by the heat to be a metal vapor as a laser medium. The metal atoms in the metal vapor are subjected to the pulse discharge, then excited to form the population inversion. Thus, laser oscillation is effected through the windows 16a, 16b by an optical cavity comprising the total reflection mirror 17 and the output mirror 18 disposed at both the outsides of the windows 16a, 16b, thereby outputting a laser beam from the output mirror 17.
However, in the conventional metal-vapor laser apparatus, the temperature of the interior of the discharge tube 1 becomes extremely high when the metal vapor is generated. Moreover, the anode 7 and the cathode 8 are integrally secured by screws 10 to the current introducing plate 9a, 9b together with the discharge tube 1, and materials of the discharge tube 1 and the anode 7 or the cathode 8 are different from each other.
Thus, the difference of the thermal expansion coefficient between the discharge tube 1 and both the electrodes 7 and 8 causes warp of the two electrodes and degrades the parallelism between them. As a results, the discharge becomes unstable and the output of the laser oscillation becomes much degraded by concentration of the discharge.
Moreover, in the conventional metal-vapor laser apparatus of a transverse exciting type, when discharge of a high current density is effected under a high gas pressure between the anode 7 and the cathode 8, the discharge is likely to be concentrated at a particular portion, so that only relatively small output can be obtained.
While, when some metal salts, such as chlorides or bromides, are used in place of the lumps of metal as a source for generating the metal vapor, the temperature in the discharge tube 1 can be decreased at some extent. However, when a halogen gas of chlorine or bromine is generated in the discharge tube 1, the discharge becomes very unstable so that stable laser oscillation can not be obtained.